The present invention relates to a process for the manufacture of high performance polyimide hollow fibres. Specifically, the invention enables simple manufacture of high performance polyimide gas separation membranes using environmentally friendly solvents for the preparation of the dope solutions, bore fluids and coagulation solutions of hollow fibre manufacturing processes.
Polyimides are known to be useful for the manufacture of membrane materials. In particular, fluorinated polyimides containing 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) as a monomer component have been identified as promising membrane materials for their high gas permeability and selectivity due to their rigid primary structures with bulky CF3 groups, which tend not only to inhibit efficient packing of polymer chains but also reduce local segmental mobility. For example, 6FDA-2,6-DAT polyimide (poly(2,6-toluene-2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane diimide), was reported to possess excellent intrinsic gas separation properties with a selectivity of CO2 over CH4 around 45. 6FDA-based polyimides also exhibit many other desirable properties such as spinnability, thermal and chemical stability and mechanical strength. These important properties make the commercial applications of these membranes for gas separation practical.
Asymmetric hollow fibres are one of the most preferred membrane morphologies for gas separation due to their high ratio of membrane surface area to volume. The ideal asymmetric hollow fibre membranes should consist of a defect-free ultra-thin skin layer supported by a porous substructure layer. But it is still a challenge to fabricate hollow fibre membranes with a completely defect-free ultra-thin dense skin layer due to the complexity of the formation mechanism and process of hollow fibre spinning.
In terms of preparing asymmetric hollow fibre membranes, it is important to choose an appropriate spinning system and proper spinning conditions. Usually, the spinning system is a ternary system with one polymer, one solvent and one non-solvent. However, a binary (one polymer and one solvent) system has proved workable for the preparation of hollow fibres. Thus, the addition of a non-solvent into spinning dopes is not a pre-condition to form ultrathin skin layer hollow fibre membranes.
U.S. Pat. No. 5,674,629 (to Institut Francais du Petrole) reports that hollow fiber membranes spun from a polyimide containing 6FDA-2,6-DAT using a 1,4-dioxane/N-methyl-pyrollidone(NMP) dope solvent have impressive properties. Pure gas tests conducted at 118 psi show a permeance of 162 GPU (1 GPU=1×10−6 cm3 (STP)/cm2·s·cmHg) for CO2 and a selectivity of 74 for CO2/CH4, whereas mixed gas tests at 368 psi exhibit a permeance of 155 GPU for CO2 and a selectivity of 59 for CO2/CH4. However, the spinning system and conditions reported in this patent are unsuitable for scale-up. This is due to the following facts: (1) 6FDA-2,6DAT polyimide does not dissolve well in the solvent mixture 1,4-dioxane and NMP. Thus, it is not easy to prepare a fully dissolved solution. (2) The bore fluid is made up of a mixture of acetic acid, water and 1,4-dioxane. (3) The external coagulant comprises a 52 wt % acetic acid aqueous solution, which has to be maintained during the experiment. Acetic acid is a corrosive solvent, while 1,4-dioxane is a toxic solvent. Both are not desirable for fiber spinning on a large scale because of these properties.
It is an object of the present invention to develop a process for the manufacture of polyimide hollow fibers which avoids the use of inconvenient and environmentally unfriendly solvents but which maintains good levels of performance in the resultant fibers.